Approach alarm system with unwanted signal elimination



S. M. BAG NO Oct. 16, 1956 APPROACH ALARM SYSTEM WITH UNWANTED SIGNALELIMINATION Filed Aug. 5, 1953 RNEYS INVENTOR far/14061 M- 5464 0 UnitedStates Patent APPROACH ALARM SYSTEM-WITH UNWANTED SIGNAL ELIMINATIONSamuel M. Baguo, Astoria, N. Y., assignor, by mesue assignments, toWalter Kidde & Company, Inc., Belleville, N. J., a corporation of NewYork Application August 3, 1953, Serial No. 371,854

8 Claims. (Cl. 340-258) The present invention relates to an alarm systemadapted to detect certain occurrences and so designed as to preventother occurrences, which might give to spurious signals, from causingthe alarm to be actuated. It is here specifically disclosed as embodiedin a motion detecting system operating on the Doppler principle, of thetype described and claimed in my application Ser. No. 776,368, filedSeptember 26, 1947, and entitled Method and Apparatus for DetectingMotion in a Confined Space, now Patent No. 2,655,645. It will beunderstood, however, that the subject matter of this application may beused in systems other than those disclosed in the aforementionedapplication.

The method and apparatus of the aforementioned application constitutes aburglar and fire alarm which is actuated by reason of the fact thatmotion of an intruder in a given space or the existence in such a spaceof a thermal disturbance, be it fire, an overheated radiator, or thelike, when such a space is filled with vibrations of a predeterminedfrequency or frequencies, preferably of supersonic value, will give riseto the existence, in such a space, of effective frequencies of vibrationdiffering from the predetermined frequencies and normally differing fromsaid predetermined frequencies by an amount of a different order ofmagnitude from said predetermined frequencies.

When those sound vibrations of different frequencies are converted intoelectrical vibrations of corresponding frequencies, and when thoseelectrical -vibrations are passed through a detecting apparatus capableof distinguishing between the frequencies and of actuating some controlmechanism, such as a switch which controls an alarm, on the basis of thepresence or absence of such differing frequencies, an operable alarmsystem is produced.

The utility of an alarm system is determined not only by its reliabilityin indicating each time that a certain event takes place, but also byits freedom from false alarms. The system disclosed in theaforementioned application, while it has functioned entirelysatisfactorily from a commercial point of view and While it has met witha very favorable response, has nevertheless been subject to false alarmsoccurring with a frequency which was undesirable although far fromprohibitive. These false alarms arose from a variety of transientphenomena which are usually unrelated to the security of the promisesbeing protected. A radical change in the line voltage fed to theapparatus, momentary interruptions in such line voltage, shock soundssuch as those due to water hammer in radiator pipes, or sparking ofelectrical contact have all tended to cause false alarms from time totime. It is to eliminate the effect of these and similar factors withoutdetracting from the inherent sensitivity and reliability of operation ofthe basic system that the present invention is primarily directed.

' Percussive shock sounds, such as water hammer or the like, hammerblows outside the protected premises the sound of which is transmittedto the protected premice ises by various structural elements, and thesounds produced by structural elements as they contract or expand inaccordance with changes in temperature, ultimately result in vibrationshaving frequencies comparable to those produced by an actual intruder.These sounds, when detected, and if of sufiicient magnitude, willtherefore tend to actuate the system of the aforementioned applicationto set oif the alarm. One disclosed modification of that system wascapable of eliminating the effect of shock sounds, but with aninsufiicient degree of reliability.

Another problem arose from the fact that, unless elabovate ann expensiveprecautions were taken, the contacts of the relay which control thealarm would tend to spark when the relay opened. These sparks wouldgenerate radiations which would, through electromagnetic interaction, bereproduced in the detecting circuit and give rise to a signal ofsufficient amplitude and proper frequency to cause the alarm to go rofi.Thus, after the alarm had been once actuated and then deactuated, thesystem would sometimes become inoperative-the spark accompanying theinitial normal de-actuation would cause an unwanted actuation, thiswould be followed by a de-actuation, another unwanted actuation wouldresult, and so on. The relay would continuously recycle and render thesystem useless un-ti-l steps were taken to eliminate the effect. If theonly time that the alarm were set off were to correspond to the actualdetection of an intruder, this might not be too undesirable, since thesupervisory personnel, when they came to the protected premises toinvestigate the alarm, could terminate this malfunctioning. However,systems of the type under discussion are often connected to a centralstation which periodically tests the security of the apparatus bysubjecting it to a simulated intruder. An apparatus which wouldcontinuously recycle, and thus be useless, after each such test wouldnot long be tolerated. Moreover, it is usually the case that a pluralityof these systems are all connected to the same central station andsparking of the relay contacts of one system is often transmitted to thesystems in other locations by the telephone lines linking the protectedpremises, so that the actuation of one alarm sometimes has the effect ofsetting off alarms in other locations and making it difiicult for thecentral station to determine the location of the actual intruder.

Static of other sorts also can be carried to a given unit by thetelephone lines so as to cause the alarm of that unit to be actuated atan improper time.

The sensitivity of the system of the aforementioned application toradical changes in the voltage of the operating current or to momentaryinterruptions of that current arose from the fact that, in accordancewith the normal requirements, the system had to fail safe, that is tosay, had to give an alarm if power failure occurred. It fulfilled thisrequirement exceedingly well, so well indeed that it would give an alarmfor transient conditions as well as for steady state conditions.Alarmactuating situations comparable to those produced by momentaryinterruption of the power supply have also been traced to intermittentelectrical component in the circuit.

One characteristic of all of these causes of trouble is that theygenerally do not last very long. However, their intensity ordinarily isso great as to compensate and thus produce in the detecting circuit anultimate signal of suifi-cient strength to cause the alarm to go off. Ithas been found that all of these problems can be virtually eliminatedwithout any appreciable sacrifice in the essential reliability andsensitivity \of the system by providing means in the system fordistinguishing between highly transitory phenomena and phenomena oflonger duration which arise from movement of an intruder. I havediscovered that this can be done in a surprisingly simple manner. Thesignal Which sets off the alarm is composed of a series of electricalpulses, either uni-direc-' ti-onal or alterna ing. The prior system wascapable of distinguishing between pulses the frequency of which camewithin a certain range and pulses having other frequencies, but thetransient phenomena under discussion gave rise to signals of the properfrequency. If to frequency discrimination is added numerical pulsedeterminationcounting the actual number of individual pulses which occurin a predetermined period of time in dependently of their amplitude, theproblem is solved. By so controlling the actuation of the alarm that itis set off only if at least *a predetermined number of pulses arereceived and detected Within a predetermined period of time, all of theabove disadvantages may be minimized or entirely avoided.

It must be borne in mind that more is here involved than merelyintegrating the signal pulses. Integration is partly a function of thenumber of pulses received Within a predetermined period of time, but itis also a function of the magnitude of those pulses. Hence if a certainnumber of pulses of normal magnitude Within a given period of time willcause the alarm to go off, a smaller number of pulses of greatermagnitude Within the same period of time would have the same effectinsofar as integration is concerned. Since shock sounds and signals dueto sparking are often quite extreme in magnitude, integration does notreliably eliminate them us adverse factors. It may be noted in thisregard that the system of :the aforementioned application includedintegration, but that these troubles were none the less prescut to adegree.

in order to eliminate the effect of the magnitude of the pulses in thesystem here disclosed, the integrating circuit is preceded by a circuithaving an amplitude-limiting feature. Such a circuit has thecharacteristic that so long as its input signals have a predeterminedminimum amplitude its output signals will have substantially the samemagnitude, no matter how high above the minimum the input signals maygo. Circuits having this characteristic have long been known and maytake a number of forms, and I here make no claim to the specific type ofany such circuit. tilde-limiting circuit in, conjunction with anintegrating circuit, the alarm system becomes substantially independentof transient efiects. Shock sounds will not set off the alarm unlessthey occur with great frequency and with practically no intervaltherebetween. Sparking, unless long continued and substantiallycontinuous, will not set it 'off. Even momentary interruptions in thecircuit :or in the power supply will not set it off. The use of thelimiter circuit also renders the system much more insensitive tovariations in line voltage than has heretofore been the case.

Of course, the use of the limiter-integrator circuit combination is notthe only way in which the number of pulses received within a givenperiod of time can be determined independently of their magnitude, or atleast independently of any excess of their magnitude above apredetermined normal value. For example, a time sensitive electroniccounter of any conventional type could be employed to count the numberof pulses continuously received and to permit the alarm to be actuatedonly after a predetermined number of pulses had been counted, provisionbeing made to start counting :all over again after passage of apredetermined time or after detection of a discontinuity in a series ofpulses.

To the accomplishment of the above, and to such other objects 318 mayhereinafter appear, the present invention relates to an alarm system inwhich means are provided for distinguishing between real and spurioussignals and controlling the alarm correspondingly, as defined in theHowever, by utilizing such an ampli- 4 following claims and 'asdescribed in this specification, taken together with the accompanyingdrawings in which:

Fig. 1 is a circuit diagram of one embodiment of the present invention;and

Fig. 2 is a simplified fragmentary circuit diagram of another embodimentthereof.

In general the system in connection with which the instant invention isspecifically illustrated comprises a transmitter generally designated 2adapted to be energized by an oscillator 4 so as to transmit vibrationsof a predetermined frequency of, for example 19 kilocycles per second,into a confined space. These radiations are reflected from. wells andobjects in the room and eventually impinge upon a receiver 6 whichtransforms the received radiations into an electrical signal fluctuatingat \a corresponding frequency. These signals are amplified in anamplifier section 8 and are then caused to beat with the originaltransmitted frequency in la detecting circuit portion 9 so that, if thereceived radiations differ in frequency from. the transmittedradiations, as will be the case if there is movement Within the confinedspace, a signal is produced having a frequency equal to the differencebetween the received and transmitted frequencies. The frequency of thissignal is. fairly low, the system usually being designed to detect suchfrequencies within the range of approximately 3 and cycles per second.These frequencies will hereinafter be referred to as low frequencies,the transmitted and received frequencies being referred to as highfrequencies. These terms are relative, and it will be appreciated thattheir actual magnitudes may be widely varied. For example, the highfrequency could be in the radio frequency spectrum, and the lowfrequency would have .a much higher range than that specified.

The low frequency signal is amplified, in the embodiment herespecifically disclosed passing through the same amplifier section 3, andis then fed to an amplitudelimiting circuit generally designated 19, theoutput of which will have substantially the same frequency as the inputbut the amplitude of the output of which will not substantially exceed agiven value no matter how much the amplitude of the low frequency pulsesfed thereinto may exceed a normal predetermined value. Of course, weakinput signals may produce correspondingly Weak output signals. Theimportant point is that there is an upper limit on the amplitudev of theoutput signals.

The output from the amplitude-limiting circuit ill is fed to anintegrator circuit generally designated 12, where it is integrated withrespect to time. The output of the integrating circuit 12 is connectedto a control circuit generally designated 14 which as here disclosed isbiassensitive and which is biased by the integrating circuit 112. Thecontrol circuit. 14 is operatively connected to a relay generallydesignated 16 which controls the operation of an alarm of any desiredtype, generally designated 7.8.

The nature of the control circuit 14 is such that it will cause therelay 16 to close and actuate the alarm 18 only when a bias ofpredetermined amount is applied thereto. The integrating circuit 12 andlimiter circuit it) are so designed that such a bias will be producedonly when a predetermined number of impulses are received from the lowfrequency signal within a given period of time, and when those impulseshave a predetermined minimal magnitude. The amplitude-limiting circuitit serves to ensure that any excess in magnitude of the impulses in thelow frequency signal over and above this normal minimal magnitude willhave no effect on the control circuit 1.4.

Having reference now specifically to the circuit of Pig. 1,, the basictransmitted frequency of approximately 19 kilocycles per second isgenerated by the oscillator circuit 4, defined by the triode 20 (half ofa 6NS7 tube) the frequency of output of which is determined by thecenter tapped primary of transformer 21 across which the .0015 mfd.condenser 22. is connected, the output of the tube 20 being fed back tothe grid thereof by means of .001 mfd. condenser 24 and 22,000 ohmresistor 26. A grid bias resistor 28 of 47,000 ohms is connected betweenthe grid and the cathode. The transformer 21 steps down the output ofthe oscillator to approximately 6 volts, that output being fed totransmitter 2 by means of shielded leads 30 and 32.

The receiving transducer 6 picks up radiations impinging thereon andconverts them into corresponding electrical fluctuations which are fedto the primary of transformer 31 which functions as a step-uptransformer and the secondary of which is tuned by means of the 200mmfd. condenser 34. The output of this tuned circuit is coupled to thegrid of tube 36 (one half of a 6SL7 tube) by means of 200 mmfd.condenser 38, a 220,000 ohm grid resistor 40 leading to ground viamegohm bias resistor 42, the latter being bypassed for high frequenciesby .01 mfd. condenser 44 and .05 mfd. condenser 46. The output of tube36 is coupled to the grid of tube 48 (the other half of the 6SL7 tube)by means of .01 mfd. condenser 50 and grid bias potentiometer 52, thelatter having a resistance of l megohm and having a movable tap 54bypassed by 500 mmfd. condenser 56. Because of the low impedance of thecondenser 56 to the high frequency signal, the position of the tap 54along the potentiometer 52 will have little effect on the magnitude ofthat signal, except when the tap 54 is very close to the low end of thepotentiometer 52, at which time the impedance from the tap 54 to groundapproaches the impedance of condenser 56, thus causing the highfrequency signal to be attenuated. The primary function of the tap 54,as will become apparent later in the explanation, is to control theamplitude of the detected low frequency signal and thus to control thesensitivity of the unit to the amplitude of detected Doppler frequencyeffects.

The high frequency signal will be amplified by tube 48, whose bias isderived from the cathode biasing resistor 58 of 4,700 ohms, highfrequency bypassed by .1 mfd. condenser 60. The 510,000 ohm resistor 62and the .01 mfd. condenser 64 constitute a low pass filter which willnot permit high frequency components to pass. The high frequency signalis coupled by 500 mmfd. condenser 66 to a high pass filter defined by510,000 ohm resistors 68 and 70 and 500 mmfd. condensers 72 and 74,together with 470,000 ohm resistor 76. A selenium rectifier 78 isconnected across resistor 76 and, by shunt rectification, detects thehigh frequency signal amplified by tube 48 and passed by the high passfilter. At the same time, a portion of high frequency'output from theoscillator 4 is coupled thereto by means of 10 mmfd. condenser 80, thuscausing the received high frequency signal to beat against the signaltransmitted by the speaker unit, and if it received a signal at afrequency different from the transmitted signal, to produce a lowfrequency signal having a frequency equal to the beat or differencebetween the received and transmitted frequencies.

The frequency of such variations in the received signal caused by anintruder or by fire may range from between 3 and 180 cycles per secondin a normal burglar alarm installation. In order to prevent spuriousalarms, such as might be caused by convection from normally operatingradiators, it is preferred to cause the system to be actuated only whenfrequencies in a range between about and 100 cycles per second aredetected. The low pass filter consisting of 220,000 ohm resistors 82 and84, together with .01 mfd. condenser 86 and .05 mfd. condenser 46,permits the low frequency signal to pass therethrough, that signal beingcoupled to the grid of tube 36 by condenser 44 through resistor 40. Thelow frequency signal is then amplified by the same tube 36 which hadpreviously amplified the high frequency signal. This simultaneousamplifying of low and high frequencies by the same tube is known asreflexing and it will be seen that both tubes 36 and 48 thereforeoperate as a two-stage reflex amplifier, The low frequency, when itreaches the potentiometer 52, is not bypassed by the condenser 56 andhence the setting of the tap 54 along the potentiometer 52 will controlthe amplitude of the low frequency signal applied to the grid of theamplifier tube 48. Because the cathode bypass condenser 60 for the tube48 has a value of .1 mfd., this stage of amplification is degenerativeat the low frequencies only. The amplified low frequency signal is takenoff the plate of the tube 48 by means of the low pass filter defined bythe resistor 62 and condenser 64. The low frequency signal is blockedfrom passing through the condenser 66 because of the high passcharacteristics of the filter 6876. The low frequency signal is coupledby .01 mfd. condenser 88 and 1.2 megohm resistor 90 to the grid ofamplifying tube 92 (one half of a second 6SL7 tube), that .tubedegeneratively cathode biased by 4700 ohm resistor 94.

Plate potential is applied to the tubes 36, 48 and 92 from B+ terminal96 via 270,000 ohm resistor 98, 120,000 ohm resistor 100, 180,000 ohmresistor 102 and 270,000 ohm resistor 104, the plate volt-age beingbypassed to ground by 10 mfd. condenser 106.

The output of the amplifier tube 92, which consists exclusively of thelow frequency if detected, is coupled to the grid of amplitude limitertube 108 (the other half of the 6NS7 tube) by means of .01 mfd.condenser 110 and 2.2 megohm resistor 112. The plate of the tube 108 isconnected to the B+ terminal 114 via 1.2 megohm resistor 116, and isalso connected to ground via 160,000 ohm resistor-118. The resistors 116and 118 define a voltage divider by means of which the plate of the tube108 is operated at a voltage considerably lower than the volt-- age atthe B+ terminal 114. For example, if the voltage at 114 is approximately280 volts, the voltage at which the plate of tube 108 is operated willbe approximately 25 volts. The A. C. output voltage from the tube 108 islimited by the 25-volt plate voltage and cannot exceed it no matter howlarge the input. The limiter is stabilized by the degenerative action of3300 ohm cathode biased resistor 120. Hence, except for very smallsignals, the amplitude of the output of the limiter circuit 10 issubstantially constant and independent of the amplitude of the lowfrequency signal applied to the grid of tube 108.

The low frequency output from the limiter circuit 10 is coupled by .1mfd. condenser 122 to selenium rectifier 124 and a 1.2 megohm resistor126 connected in parallel. The rectifier 124 acts as a shunt rectifieracross resistor 126. During the positive half of the cycle, while therectifier is conducting, it shuts out the input signal. During thenegative half of the cycle, the rectifier is nonconducting and thevoltage across it becomes equal to the input. As a result, there is anaverage negative voltage across the rectifier which shows up as a seriesof D. C. negative peaks. Therefore low frequency signal caused by themotion of an intruder or by fire is con verted by rectifier 124 into aseries of negative pulses, one pulse for each cycle of the low frequencysignal. These negative pulses are fed to an integrating circuitconsisting of 1.2 megohm resistor 128 and .5 mfd. condenser 130, thejunction point 132 therebetween being connected to the grid of tube 133(the other half of the second mentioned 6SL7 tube), which forms a partof the control circuit 14. Since the resistor 128 limits the chargingcurrent, the .5 mfd. condenser must integrate the charge it receivesfrom the rectifier 124 over several cycles before it becomessufiiciently negative to operate the control circuit.

The average plate voltage of the limiter tube 108, filtered by 10 megohmresistor 134 and capacitor 130, is applied to the selenium rectifier 124through resistor 128. When there is no low frequency signal, there is nonegative voltage developed by rectifier 124 and the positive thresholdvoltage is applied to the grid of the relay tube 14 through the resistor134 from the plate of the limiter tube 108. Low frequency signalsreaching rectifier 124 via limiter tube 108 and condenser 122 will berectified.

7 and integrated by the resistor 128-con'clenser 130 combination untilthey are large enough to overcome the positive threshold bias on thegrid of tube 14. Thus minor signals caused by small amounts of airturbulence or circuit noise will not affect the plate circuit of therelay tube 14. Only if the magnitude of the low frequency signal isgreat enough will any negative grid control take place. Thenrectification will tend to build up a negative charge on the upper endof condenser 130 and thus bias the grid of tube 14 negatively so as todecrease its plate current.

The charge on condenser 130 will tend to leak 0135 via resistors 126 and128, the rate of leakage being determined by the magnitude of theselatter components. The integrating circuit will therefore have acharacteristic and predetermined time-charge characteristic. If aninsufiicient number of cycles of low frequency signal are rectified byrectifier 124 within a given period of time, the charge on condenser 130will not be built up sufiiciently to overcome the positive bias normallyapplied thereto and bring it to a sufliciently negative point. It isonly when a predetermined number of such cycles of low frequency signalare rectified that the positive bias on the grid of tube 14 will beovercome and the grid made sufliciently negative to operate the relay,and, it will be noted, because of the interposition of the limitercircuit 10 ahead of the integrator circuit 12, the latter will not beaffected by variations in the amplitude of the .low frequency signalabove a certain predetermined normal value.

The alarm relay coil 136 is connected in the plate circuit of tube 14,plate voltage being supplied from B-]- terminal 137. Shunted across coil136 are 10 mfd. condenser 138 and 15,000 ohm resistor 140 in parallel,the former acting as a filter condenser to prevent chatter, and thelatter acting as a protective resistor to prevent condenser 13% frombeing subjected to overvoltage should the relay coil 136 be opened orthe relay removed.

The tube 14 to the grid of which a positive bias is normally applied,therefore normally passes current. The relay coil 136 is normallyenergized, and the relay armature 142 is normally engaged with blindfixed contact144. The fixed contact 146, with which the armature 142makes contact when the relay coil 136 is de-energized, is connected tobattery 148 and then to ground via an alarm instrumentality 18 which maytake any desired form and which is here specifically disclosed as a bell150. It will therefore be appreciated that so long as the tube 14 passesa sufficient amount of current the circuit through the alarminstrumentality '18 will be open, but whenever the relay coil 136 isinsufliciently energized, as will be the case when a suificientlynegative bias is applied to the grid of the tube 133 by the integratingcircuit 12, the armature 142 will engage the fixed contact 146 (as shownin Fig. l), the circuit through the instrumentality 18 will be closed,and the alarm will be set off.

By choice of values of circuit components, any desired operatingconditions can be achieved. With the circuit components herespecifically disclosed, it has been found that approximately pulses persecond of normal magnitude received by the integrator circuit 12 willcause the alarm to go oh", but any fewer number of pulses per secondwill not cause the alarm to go off. From a practical point of view, thismeans that one shock sound per secend will not set off the alarm butthat one shock sound every tenth second will set it 'otf. Shock soundsof the latter frequency are so rare that they warrant investigation, andconsequently this is not deemed to be a drawback. This particularadjustment means that movement of an intruder of approximately 3 /2inches in a second will still cause the alarm to go off, and thislimitation is entirely practical.

The amplitude limiting circuit it) in the system of Fig. 1 operates onthe principle of reduction of the voltage applied to the plate of thelimiter tube 108. Fig. 2 illustrates an alternative arrangement in whichthe voltage to the grid of limiter tube 168' is rendered substan- 2%tially insensitive to variations in magnitude of the low frequencysignal above a predetermined normal value by connecting a pair ofselenium rectifiers I152, 154 in parallel across the grid circuit andactive in opposite directions. The characteristics of such a network aresuch that the amplitude of the voltage output varies with the amplitudeof the voltage input only over the lowest values of voltage inputamplitude, after which the voltage output amplitude will besubstantially constant no matter how high the voltage input amplitudemay go. This is to be considered merely exemplary of numerousamplitude-limiting arrangements which might be used.

By the same token, certain specific integrating circuits are disclosed,and many other types of such circuits are known and may be used.

it will be appreciated that when, in the specification and claims,signals or variations in voltage or current are referred to in terms ofpulses, that term is broad enough to include pulses of widely varyingwave shapes and also pulses which alternate in direction, as in the caseof alternating current.

By the arrangement above described the system has been renderedsubstantially insensitive to variations in line voltage and has beenenabled to differentiate between transient conditions and continuingconditions, the term transient being employed in the sense that thenumber of signal pulses produced in a given period of time is below thatnumber required to set oii the alarm. The effectiveness of the instantinvention can be evaluated not only from the fact that shock sounds,sparking and static no longer produce false alarms, but also mm the factthat the plug connecting the unit to its source of power may be pulledout and pushed back into the socket quickly a number of times withoutcausing the alarm to go oil, whereas if the plug is held out of thesocket for an appreciable period of time the alarm will go off.

While but two embodiments of the invention have been here specificallydisclosed, it will be apparent that many variations may be made thereinwithout departing from the spirit of the following claims.

I claim:

1. In a motion detecting system comprising means for transmittingradiations of a given frequency into a space to be protected, means forreceiving said radiations after they have traversed said space, andmeans for detecting variations in the frequency of said receivedradiations when compared with said transmitted radiations and indicatingthe presence or absence of motion in said space in accordance with thepresence or absence of said detected variations; the improvement whichcomprises said indicating means being actuated in accordance with thepresence or absence of a predetermined degree of electrical bias appliedthereto, said detecting means comprising means for transforming saiddetected frequency variations, if present, into a series of pulses,

' means operatively connecting said pulses to an amplitude limitingcircuit the output of which is substantially uniform in amplitudeindependently of variations in the amplitude of its input above a normalvalue and operatively connected to receive said pulses, and anintegrating circuit, the output of said limiting circuit beingelectrically connected to said indicating means via said integratingcircuit to bias said indicating means, whereby the attainment of thepredetermined degree of electrical bias sufficient to cause saiddetecting means to indicate the presence of motion within said space isdetermined by the number of pulses reaching said integrating circuitwithin a predetermined period of time and substantially independent ofpossible excess in amplitude of said pulses.

2. In the motion detecting system of claim 1, means constantly biasingsaid indicating means in a sense opposite to the bias developed by saidintegrating circuit,

the bias produced by said integrating circuit therefore having toovercome said bias before said detecting means can be biased to indicatethe presence of motion.

3, The motion detecting system of claim 1, in which the pulse output ofsaid detecting means is alternating in character and in Which saidintegrating'circuit cornprises a rectifying means in parallel with acondenser, a point of junction between said rectifying means and saidcondenser being connected to said indicating means to bias the latter.

4. In an alarm system or the like, an alarm, control means operativelyconnected to said alarm and sensitive to the presence or absence of apredetermined degree of electrical bias applied thereto, means includinga receiver for detecting the occurrence of a given event and translating said detection into an output defined by an alternating series ofelectrical pulses, a limiting circuit, the output of which issubstantially uniform in amplitude independently of variations in theamplitude in its input above a normal value, the output of saiddetecting means being electrically connected to and defining the inputto said limiting circuit, and an integrating circuit electricallyconnected to the output of said limiting circuit and comprising arectifying means in parallel with a condenser, a point of junctionbetween said rectifying means and said condenser being electricallyconnected to said alarm control means to bias said control means to analarm-actuating condition, whereby the attainment of the predetermineddegree of the electrical bias sufiicient to cause said control means toactuate said alarm is determined by a minimum number of pulses of theoutput of said receiver which reach said integrating circuit within agiven period of time and is substantially independent of possible excessin amplitude of said pulses.

5. In a motion detecting system comprising means for transmittingradiations of a given frequency into a space to be protected, means forreceiving said radiations after they have traversed said space, :andmeans for detecting variations in the frequency of said receivedradiations when compared with said transmitted radiations and indicatingthe presence or absence of motion in said space in accordance with thepresence or absence of said detected variations; the improvement whichcomprises said detecting means comprising means for translating saidvariations into a series of electrical pulses, a limiting circuit, theoutput of which is substantially uniform in amplitude independently ofvariations in the amplitude in its input above a normal value, saidpulses being fed to and defining the input to said limiting circuit, andan integrating circuit, the output of said limiting circuit beingelectrically connected to said indicating means via said integratingcircuit, said indicating means being so 10 constructed and arranged asto indicate the presence of motion within said space in accordance withthe reception by said integrating circuit of a predetermined number ofpulses within a given period of time, said indicating means thereforebeing actuated substantially independently of possible excess inamplitude of said pulses.

6. In the system of claim 5, means constantly biasing said indicatingmeans away from motion-indicating condition, the output from saidintegrating circuit acting in opposition thereto and having to overcomesaid constant bias before said indicating means will indicate thepresence of motion.

7. In a motion detecting system comprising means for transmittingradiations of a given frequency into a space to be protected, means forreceiving said radiations after they have traversed said space, andmeans for detecting variations in the frequency of said receivedradiations when compared with said transmitted radiations and indicatingthe presence or absence of motion in said space in accordance With thepresence or absence of said detected variations; the improvement whichcomprises said detecting means comprising means constructed and arrangedfor counting the number of pulses of said variations, if present, whichare detected within a given period of time substantially without regardto excess of the amplitude of said pulses above a normal value, saidcounting means being operatively connected to said detecting means andactuating the latter only when a predetermined number of pulses arecounted Within a given period of time, said counting means comprising anamplitude limiting circuit to which the detected variations are fed, theamplitude of the output of said limiting circuit being substantiallyindependent of variations in the amplitude of its input above a normalvalue, and wherein an integrating circuit is connected in series withsaid limiting circuit, said indicating means including a bias-sensitiveinstrumentality operatively connected to the output of said integratingcircuit and biased thereby.

8. In the motion detecting system of claim 7, means to apply a constantbias to said instrumentality in a sense opposite to the bias developedby said integrating circuit.

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